To make a post on someone else's thread, just type what you want to say in the message box at the end of that thread and click on "Post Quick Reply" at the bottom of the page. It's always a good idea to check over your post before actually putting it up on the thread..to do that, you can click on "Go Advanced" and "Preview Post". Once you've proof read what you've typed, click on "Submit Reply".
Your first few posts will not show up immediately, but should appear within a few hours. No need to try to repost your message--just hang tight and it will show up.